One such permanent magnet arrangement is disclosed in the following publication: IEEE Transactions on Magnetics, Vol. 44, No. 8, August 2008, pages 2009 to 2015, Using Modular Poles for Shape Optimization of Flux Density Distribution in Permanent-Magnet Machines, A. H. Isfahani, S. Vaez-Zadeh, and M. A. Rahman.
This permanent magnet arrangement is shown in FIG. 1 of the accompanying drawings. FIG. 1 also shows schematically the flux density distribution of the magnetic field produced by the arrangement.
FIG. 1 shows a pair of adjacent permanent magnet poles 1a, 1b, each pole comprising a central, main, high energy-product magnet 2a, 2b, and, to either side of the magnet 2a, 2b, side, subsidiary, low energy-product magnets 3a, 3b, 3c, 3d. Each pole 1a, 1b has a base 4a, 4b, top face 5a, 5b, and sides 6a, 6b, 6c, 6d. The arrows on the magnets 2a, 2b, 3a, 3b, 3c, 3d represent the direction of magnetization of the magnets. Each magnet 2a, 3a, 3b of pole 1a has a direction of magnetization directed perpendicularly away from the base 4a of the pole. Each magnet 2b, 3c, 3d of adjacent pole 1b has a direction of magnetization directed perpendicularly towards the base 4b of the pole.
The pair of adjacent poles 1a, 1b produces a magnetic field having a flux density distribution as shown. For each pole 1a, 1b, the distribution has a central, main region 2a′, 2b′ of relatively high flux density corresponding to the stronger high energy-product magnet 2a, 2b of the pole, and side, subsidiary regions 3a′, 3b′, 3c′, 3d′ of relatively low flux density corresponding respectively to the weaker low energy-product magnets 3a, 3b, 3c, 3d of the pole.